1. Field of the Invention
The present invention relates to a combined cycle electric power plant, and more particularly to a control of the operation of the plant's steam generator and of its transfer thereof from a WET mode to a DRY mode of operation.
2. State of the Prior Art
In the design of modern electric power plants, it is a significant object to achieve the greatest efficiency possible in the generation of electricity. To this end, steam generators are designed to extract heat efficiently from and to use the extracted heat to convert a fluid such as water into superheated steam at a relatively high pressure. Further, such steam generators have been incorporated into combined cycle electric generating plants including both gas and steam turbines wherein the exhaust gases of a gas turbine are used to heat water into steam, then to be transferred to the steam turbine. Typically, steam generators include a water heating section or economizer tube, a high pressure evaporator tube and finally a superheater tube, whereby water is gradually heated while increasing levels of pressure applied thereto to provide from the superheater tube, superheated steam to be supplied to the steam turbine. A condenser is associated with the steam turbine to receive the spent steam therefrom and to convert it into water condensate to be fed back to the steam generator.
In a combined cycle electric power plant, the steam turbine is combined with a gas turbine, whereby the heated exhaust gases of the gas turbine, otherwise lost to the atmosphere, are used to heat the circulated fluid and to convert it into steam to drive the steam turbine. In this manner, a significant reduction in the amount of fuel required to heat the steam is achieved and the heat contained in the gas turbine exhaust gases is effectively utilized. Further, an afterburner associated with the exhaust exit of the gas turbine serves to additionally heat the gas turbine exhaust gases, whereby the heat required to generate sufficient steam to meet load requirements is provided. In particular, under conditions of relatively high loads wherein the heat of the gas turbine exhaust gases is insufficient to supply the steam requirements, the afterburner is turned on to further heat the gas turbine exhaust gases.
Referring now to FIG. 1, there is shown a combined cycle electric power plant of the prior art including a gas turbine 162' whose output drives a generator 13'. The hot exhaust gases derived from the gas turbine 162' pass by way of duct 800L through a steam generator (boiler) 18' and out through a vent duct 66' into the atmosphere. The gas turbine exhaust gases are initially heated by the gas turbine 162' and then passed to the steam generator 18', whereby the heat therein may be transferred to a fluid passing therethrough.
The steam generator 18' produces a superheated steam which is supplied to a steam turbine 36' for driving a coupled generator 44'. The spent steam from the steam turbine 36' is passed through a condenser 31', which converts the spent steam into water condensate. The resulting condensate is pumped by a condensate pump 30' back to the steam generator 18' for use in making more steam.
If for some reason, such as the repair of the steam generator 18' or the steam turbine 36', the steam generator 18' or the steam turbine 36' is shut down, the gas turbine 162' may nevertheless continue to be operated by diverting its exhaust gas from the steam generator 18' to the atmosphere, by way of a bypass exhaust stack 808L. Such diversion is accomplished by a suitable diversion or gate mechanism, represented as a pivoted closure gate 806L movable from a first position shown in solid line wherein the gas turbine exhaust gases are directed to the steam generator 36', to a second position as shown in dotted line wherein the exhaust gas turbine gases are directed through the bypass duct 808L to the atmosphere. In the case where the closure gate 806L is disposed in the dotted line position as shown in FIG. B-41, no exhaust gas turbine gases pass to the steam generator 18'.
In practice, the closure mechanism represented generally by the numeral 806L in FIG. B-41, may take the form of two sets of grate structures or movable vanes, one for blocking off the passage to the steam generator 18' and the other for opening up the entrance of the bypass stack 808L. The operation of such mechanisms is cumbersome and such mechanisms have a decided tendencey to become defective.